[Libguestfs] Point-in-time snapshots (was: Re: Inspection of disk snapshots)

[Stefan Hajnoczi]

On Fri, Mar 27, 2015 at 03:35:02PM +0000, Richard W.M. Jones wrote:
> On Fri, Mar 27, 2015 at 03:21:25PM +0000, Stefan Hajnoczi wrote:
> > On Fri, Mar 27, 2015 at 12:31:41PM +0000, Richard W.M. Jones wrote:
> > > On Fri, Mar 27, 2015 at 12:15:34PM +0000, Stefan Hajnoczi wrote:
> > > > On Thu, Mar 26, 2015 at 12:02:27AM +0100, Kashyap Chamarthy wrote:
> > > > > So, something like?
> > > > > 
> > > > >   . . .
> > > > >   { 'execute': 'drive-backup', 'arguments':
> > > > >         { 'device': 'drive-virtio-disk1', 'sync': 'full', 'target':
> > > > >         'nbd://localhost', 'mode': 'absolute-paths', 'format': 'qcow2' } 
> > > > >   . . .
> > > > > 
> > > > > Same question as yours, what is the NBD server going to run?
> > > > > 
> > > > > 
> > > > > My only NBD testing so far has been with w/ NBD over Unix socket or
> > > > > over TCP[**].
> > > > 
> > > > For 'sync': 'full' mode qemu-nbd or nbd-server can be used as the
> > > > server.  You probably don't want 'format': 'qcow2', just raw data over
> > > > NBD.  That way the NBD server can implement whatever storage backend it
> > > > likes (raw, qcow2, something else).
> > > > 
> > > > For incremental backup the NBD server would either be a qemu-nbd
> > > > instance with a qcow2 image and backing file:
> > > > 
> > > >   full-backup.img <- incremental-1.qcow2 <- incremental-2.qcow2
> > > > 
> > > > Or the NBD server would be a custom backup application that does
> > > > something smart with the incoming NBD WRITE requests.
> > > 
> > > What I care about is connecting libguestfs to qemu and reading a
> > > snapshot at some point in time, even though the guest is still writing
> > > away to its disks.  Is this possible with drive-backup (or otherwise)?
> > 
> > Yes, that is what drive-backup does.
> > 
> > New writes coming from the guest are held up until the old data has been
> > written to the NBD target.
> > 
> > That way you get a point-in-time snapshot while the guest continues
> > running.
> 
> I understand how that can work for backups, where you want to copy
> a whole disk consistently.
> 
> But libguestfs doesn't want to do a backup, nor get a copy of the
> whole disk, it just wants to access a scattering of blocks (maybe a
> few hundred) but at a single point in time, in as lightweight a manner
> as possible.

There is a way to do that.

Create a temporary qcow2 backed by the disk:

guest.img <- tmp.qcow2

Now use drive-backup with sync mode 'none' so only new guest writes
cause old data to be stashed in tmp.qcow2 before it is overwritten.

Finally, use the run-time NBD server for random access to tmp.qcow2.

When you are done, kill the drive-backup job and delete tmp.qcow2.

Stefan
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